Explicit constructor taking multiple arguments

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Up until C++11, yeah, no reason to use explicit on a multi-arg constructor.

That changes in C++11, because of initializer lists. Basically, copy-initialization (but not direct initialization) with an initializer list requires that the constructor not be marked explicit.

Example:

struct Foo { Foo(int, int); };
struct Bar { explicit Bar(int, int); };

Foo f1(1, 1); // ok
Foo f2 {1, 1}; // ok
Foo f3 = {1, 1}; // ok

Bar b1(1, 1); // ok
Bar b2 {1, 1}; // ok
Bar b3 = {1, 1}; // NOT OKAY

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